The present disclosure is directed to turbomachinery and, more particularly, to turbomachine components having abrasive coatings.
Turbomachinery, such as gas turbine engines, have rotors with one or more rows of rotating blades. Radially outward tips of the blades are located in close proximity to a typically stationary surface which is, or acts as, a seal. To maximize engine efficiency, leakage of gas or other working fluid around the blade tips should be minimized. This may be achieved by configuring the blade tips and seal such that they contact each other during periods of operation of the turbomachine, such as during initial operation of the turbomachine referred to as the green run, during normal operation, and possibly during other operating conditions such as a bird strike. With such a configuration, the blade tips act as an abrading component and the seal can be provided as an abradable seal. Generally, the blade tip is harder and more abrasive than the seal. Thus, the blade tips will abrade or cut into the abradable seal during those portions of the engine operating cycle when the blade tip comes into contact with the abradable seal. This interaction between blade tips and seal is desirable as it helps to provide minimal leakage between blade tips and seal.
Since gas turbine engines, such as aircraft gas turbine engines, experience cyclic mechanical and thermal load variations during operation, their geometry varies during different stages of the operating cycle. Thus, the blade tips should retain their cutting capability over many operating cycles compensating for any progressive changes in gas turbine engine geometry.
During certain engine operating conditions, such as during a bird strike or engine surge, gas turbine engines have shown high radial interaction rates between the blade tips and abradable seals (˜40″/s) that can cause rapid depletion of the abrasive blade tip coating when rubbed against the abradable seals. Low radial interaction rates, which occur during certain engine operating conditions such as during low transient thermal or mechanical loading cycles (for example during the green run), can also result in excessive wear and damage to abradable seals through the generation of large thermal excursion within the seal system (abrasive tip and abradable seal).
If the abrasive coating on the blade tip is depleted, unwanted sliding contact or rubbing of the base material of the blade tip, such as titanium, nickel, steel, and aluminum alloys, and the abradable seal may occur. This results in direct contact between the base material of the blade tip and the abradable seal. Contact of base material with the abradable seal can cause unwanted conditions within the gas turbine engine.
An alternative blade tip and seal configuration is needed for enabling reduced clearance during normal running and other transient conditions, while addressing the above-described issues.